Emanuel's answer is correct. (Only the speculation in the last sentence is unfounded.) But that leaves the question why some German speakers would make this mistake. I think I can explain that.
It seems to me that this is an obvious example of a hypercorrection. In German, words of Latin origin eventually get c replaced by either z (before e and i) or k (before a, o, u). As a result, when replacing a German word of Latin origin by its English cognate, we have to replace z and k by c. (Zentrum -> centre, zirka -> circa, Kontakt -> contact, ...)
Horizontal superficially looks like a word of Latin origin, so it gets this treatment. Of course, the fact that the z in horizontal is before an o, not an e or i, should be clue enough that this is wrong. But in a few cases this largely automated process will trigger anyway.